1. Field of the Invention
This invention relates to apparatus for measuring the concentration of a volatile component in a gas and in particular, but not exclusively, to breath alcohol testing apparatus.
2. Description of the Prior Art
There are in general two types of breath alcohol testing apparatus; screening and evidential. For the latter the courts of most countries require that the detector for alcohol (ethanol) is re-calibrated before each breath sample is taken. One of the ways of achieving this calibration is to release into the detector a quantity of a gas containing a known concentration of ethanol from a pressurised cylinder which has been prepared in the laboratory. However, this concentration is only accurate for a single atmospheric pressure and accordingly the calibration can cause the detector to read too high or too low, depending on the altitude at which the reading is taken and the local ambient atmospheric conditions. To date attempts have been made to overcome this problem by artificially weighting the detector output in accordance with the altitude at which the reading is taken. This is unsatisfactory because the altitude is often not known accurately and the atmospheric pressure at any given altitude is not constant.
The present invention consists in apparatus for measuring the concentration of a volatile component in a gas, including a detector for detecting the volatile component and producing an output signal representing the concentration of the volatile component in the gas, a gas standard containing a predetermined concentration of the volatile component, sampling means for delivering either a test sample or a sample from the standard to the detector, means for measuring the atmospheric pressure and output means responsive to the pressure means and to the detector output signal for generating a representation of the measured concentration.
Preferably, the sampling means is arranged to deliver a standard sample to the detector prior to the delivery of each test sample.
The apparatus may have many uses, for example in the wine or chemical trades, but it is particularly suited for breath testing drivers and the like to see that they do not exceed legal limits of breath alcohol concentrations. In this case the test sample will not be subject to variation due to ambient pressure conditions and the output means may calculate the value of the measured concentration S from the following formula: ##EQU1## wherein C is the concentration of volatile component in the gas standard Vc samp is the value of the detector output signal for the test sample
Vc cal is the value of the detector output signal from the standard sample PA1 falt is the ratio of the measured pressure to a calibration pressure
The pressure sensor may measure absolute pressure.
The detector may be a fuel cell, a semiconductor or an infrared detector or indeed any other suitable detector. It may operate in conjunction with a gas chromatographic column.
Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description: